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Chromosomal Aberrations. Changes in the numbers of chromosomes Polyploidy Extra complete sets of chromosomes 3N, 4N, 5N, etc. Aneuploidy Extra or missing single chromosomes 2N + 1, 2N -1, etc. Chromosomal Aberrations. Changes in structure Changes in the number of genes
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Chromosomal Aberrations • Changes in the numbers of chromosomes • Polyploidy • Extra complete sets of chromosomes • 3N, 4N, 5N, etc. • Aneuploidy • Extra or missing single chromosomes • 2N + 1, 2N -1, etc.
Chromosomal Aberrations • Changes in structure • Changes in the number of genes • deletions: genes missing • duplications: genes added
Chromosomal Aberrations • Changes in structure • Changes in the location of genes • inversions: 180o rotation • translocations: exchange • transpositions: gene “hopping” • Robertsonian changes: fissions or fusions
Polyploidy • Having extra sets • 3N, 4N, etc. • Suffix: “-ploid” or “-ploidy” • 3N = triploid • 4N = tetraploid
Polyploidy N = A B C: 2N = AA BB CC:
Polyploidy N = A B C: 3N = AAA BBB CCC
Polyploidy • Monoploidy (haploidy): rare in animals • exceptions: Bees: males are haploid - develop from unfertilized eggs; females are diploid • More common in plants • alternation of generations increases occurrence of haploidy
Haploidy in Plants • Occasionally, unfertilized gamete may develop into adult plant • usually small, with lowered viability • sterile
3N or More in Animals • Most common form of polyploidy in animals is triploidy • arises from two sperm fertilizing the same egg • if the organism survives, it is sterile • pairing of homologues in meiosis is disrupted • Survival is extremely rare
3N or More in Plants • Polyploidy generally improves viability in plants • Plants are larger, produce larger flowers, more seeds, hardier, etc. • Pairing at meiosis is still a problem, especially w/ odd ploidies: 3N, 5N, 7N, etc. • May reproduce asexually
Autopolypoidy Extra sets of chromosomes come from the same species Arise from double fertilization usually All chromosomes have homologues Allopolyploidy Extra sets of chromosomes come from different species Arise from hybridization New chromosomes have no homologues 3N or More
Allopolyploidy or hybridization Horse + donkey mule haploid 32 31 N = 63 +
Instant Plant Speciation Through Allo- and Autopolyploidy • Possible for entirely new species of plant to be created almost instantly • Hybridization (allopolyploidy) followed by autopolyploidy --> plant w/ totally different chromosomal make up from either parent • Fertile only w/ itself; NEW SPECIES
Aneuploidy • Extra single chromosomes or missing single chromosomes • 2N + 1 • 2N - 1 • Suffix: “-somy” or “-somic” • 2N + 1 = trisomy • 2N - 1 = monosomy • 2N + 2 = tetrasomy
Aneuploidy • Generally arise through non-disjunction at meiosis • homologues or chromatids do not separate • gametes contain 2 or no copies of one chromosome
Aneuploidies in Humans • Most aneuploidies in humans lead to such drastic effects, the fetus is spontaneously aborted early in development • A few survive ‘til birth; some beyond
Trisomy 21; Down Syndrome • 47, +21 • 1/700 live births • over 60% of conceptions aborted spontaneously • 20% stillborn • incidence increases sharply w/ maternal age • 1/300 for 35 year olds • 1/22 for 45 year olds
Trisomy 21 • characteristic facial appearance • small nose, flat face, epicanthal fold • single palmar crease • mental retardation (avg. IQ < 50) • multiple complications • heart disease • leukemia • epilepsy • some fertility
Trisomy 18; Edward Syndrome • 47; +18 • 1/8000 live births; maternal age affect • low birth weight • multiple dysmorphic features • chin, ears, single palmar crease, clenched hands • malformations of the brain, heart, kidneys, and other organs • rarely survive beyond 1 year
Trisomy 13; Patau Syndrome • 47; + 13 • 1/20,000 live births; maternal age effect • multiple dysmorphic features • micropthalmia, cleft palate, clenched fists, polydactyly, ears and scalp abnormal… • heart defects; systemic defects…. • 50% die in first month; rarely survive beyond 1 year
47, XXY; Klinefelter Syndrome • 1/500 live male births (?) • often asymptomatic except for sterility, learning disabilities • small testes; low testosterone levels • poorly developed male 2o sexual charact. • some female characteristics: • enlarged breasts, elongated limbs, increased incidence of “female” diseases: breast cancer, scoliosis, osteoporosis • hormone therapy improves symptoms
47, XYY; Jacobs Syndrome • 1/1000 live male births? • formerly called “criminal chromosome” • 99% asymptomatic, though high incidence in penal institutions for the mentally subnormal (20/1000) • lower than average intelligence? (learning disabilities) above average height, tendency to severe acne
45, XO; Turner Syndrome • 1/2500 live female births • generally asymptomatic ‘til puberty • lack of 2o sexual characteristics; amenorrhoea • short stature; low hair line; • heart disease, renal malformations, ovaries generally underdeveloped, sterile • hormone therapy helpful
47; XXX • 1/700 live female births (?) • generally asymptomatic • 15 - 25% mildly mentally retarded • some sterility
Changes in Chromosome Structure • Changes in the numbers of genes • deletions • duplications • Changes in the location of genes • inversions • translocations • transpositions • Robertsonian changes
Deletions • Loss of a (generally small) segment of chromosome A B D E F G A B C D E F G C
Deletions • Arise through spontaneous breakage • some chromosomes have fragile spots • radiation, UV, chemicals, viruses may increase breakage
A B C D E F G x A B C D E F G A B C D E G A B C D E F F G Deletion Duplication Deletions • May arise through unequal crossing over
Deletions • Large deletions will most probably be lethal • Smaller deletions may allow survival • E. coli: deletions of up to 1% have been observed in living cells • D. melanogaster : deletions of up to 0.1% observed
Deletions in Humans • Cri-du-chat syndrome • Micro deletion of chromosome 5 • DiGeorge syndrome • Micro deletion of chromosome 22 • Schizophrenia & Obsessive Compulsive Disorder • Micro deletion of chromosome 22 associated
Deletions in Humans • Angelman syndrome • Micro deletion of chromosome 15 • Prader-Willi syndrome • Micro deletion of chromosome 15
lack of muscle tone in newborn poor swallowing reflex as adult - gross obesity mean I.Q. ~ 50 microdeletion of 15 developmentally delayed jerky movements stiff, fixed smile uncontrolled laughter abnormal E.E.G., epilepsy microdeletion of 15 Prader-Willi and Angelman Syndromes Prader-Willi Angelman
Duplications • Redundant segment of a chromosome A B C D E F F G A B C D E F G
Effects of Duplications on Phenotype • Some duplications have a direct effect on phenotype • “Bar” in Drosophila • Acts as an incompletely dominant allele • BB = reduced eye size • Bb = slightly reduced • bb = normal eye
Evolutionary Effects of Duplications • Allows development of new, related genes • duplicated gene may mutate and produce a new form of the gene • function may differ slightly, or time of action may differ, etc. • human hemoglobins
Human Hemoglobins • Hemoglobin is a tetramer • several different polypeptides which may be involved in forming functional hemoglobin • adults: a(1 & 2) and b polypeptides • embryonic and fetal development: d, e, g(A and G) polypeptides
Globin gene clusters a – like cluster (chr. 16): x yx ya1 a2 a1 5’ 3’ 2 Kb b – like cluster (chr. 11): yb2 eGg Ag yb1d b 5’ 3’ 4 Kb
Human Hemoglobins • a-1 and a-2: both on chr. 16; both 141 amino acids • g-A and g-G: chr. 11; 146 amino acids; differ by only one amino acid • b and d: chr. 11; 146 amino acids; differ by 10 amino acids
Human Hemoglobins Possible phylogeny: myoglobin a-1 a-2 g-A g-G bd 500 million years ago ancestral gene
normal Down Syndrome Duplication of NOR and Down Syndrome • NOR = nucleolar organizer region
Changes in the Location of Genes • Inversions • Translocations • Transpositions • Robertsonian changes No gain or loss of information; just rearrangement of genome.
180O A B C H G F E D I J K Inversions • 180o reversal of chromosome segment A B C D E F G H I J K
Inversions • Produced through breakage and reassociation of chromosome D E C B A F G
Inversions • Produced through breakage and reassociation of chromosome D E C B A F G
Inversions • May change phenotype through “position effects” • move active genes to sites generally inactive; lose gene function • move inactive genes to sites generally active; gain gene function • May act to preserve blocks of genes (specific alleles) which function well together
Inversions and Crossing-Over • Inversions cause complicated synapsis at meiosis for heterozygotes • Chromatids involved in crossing over do not allow development of functional gametes • Only parental type chromosomes passed on
E D F C A B G A B F E D C G A B C D E F G Paracentric
E D F C A B G 1 2 3 4 1 2 3 4 A B F E D C G A B C D E F G Pericentric
Inversions and Crossing-Over • Only parental type chromosomes passed on • Recombination leads to large duplications and deletions in the chromosomes • gametes do not function or zygote does not survive
Translocations • Exchange of segments between non-homologous chromosomes F E L D M N O P C B Q A